By X-ray crystallographic analyses of the substrate and some inhibitors of the enzyme dihydrofolate reductase (DHFR) it is proposed to obtain detailed information on the modes of intermolecular association in which these substances may participate. By analysis of these modes in conjunction with some conformational energy calculations and the, soon to be available, atomic coordinates of the enzyme it is hoped to gain some important insights into ways of improving either the specificity or the activity of DHFR inhibitors. Specific compounds designated for crystallographic study are the substrates folate (as Rb ion salt) and dihydrofolate (free acid) and the antimalarial DHFR inhibitor cycloguanil. Energy calculations on methotrexate and dihydrofolate are expected to yield information which will help in understanding the large difference in binding constants for these two substances.